This application claims the benefit of U.S. Provisional Application No. 60/587,758, filed Jul. 14, 2004. U.S. Provisional Application No. 60/587,758 is incorporated by reference herein in its entirety.
The following relates to the surgical and medical monitoring arts. It especially relates to monitoring of retinal temperature during laser eye surgery, and will be described with particular reference thereto. However, the invention will also find application in conjunction with other optical surgical procedures and in the monitoring of ocular tissue temperature during various ocular medical procedures and clinical ophthalmology studies.
A long-felt need in laser eye therapy is the ability to monitor the temperature of the irradiated tissue during the laser procedure. In ophthalmology laser therapy is used to treat a number of different ocular tissues and ocular pathologies, including retinal tissue and diseases. Age-related macular degeneration, which is a leading cause of poor vision in aged persons, is one pathology that can be treated using laser surgery. When a choroidal neovascularization (CNV) is present, the decrease of vision is typically more rapid and irreversible. Laser treatment allows closure of CNV; however some surrounding retinal tissue is also destroyed. Accurate control of retinal temperature during the laser treatment can reduce damage of healthy tissue during laser eye treatment. For example, one proposed treatment which reduces retinal damage is transpupillary thermotherapy (TTT). Conventional retinal photocoagulation uses brief 40° C. to 60° C. temperature increases to produce lesions that are immediately visible. In comparison, TTT uses lower (10° C.) temperature increases, but maintains them for about 60 seconds to treat CNV. In typical laser eye therapy, these temperatures are estimated based on a mathematical model of the effect of the laser on the retina. Such temperature estimates are approximate, and the actual retinal temperature varies among patients and retinal location of the treatment.
Reported complications or adverse events of TTT for neovascular age-related macular degeneration include retinal pigment epithelial (RPE) tear and retinal arteriole occlusion. Randomized, prospective controlled clinical trials are under way to compare outcomes of TTT intervention for occult CNV with the natural progression of the disease.
The increase in temperature during laser photocoagulation is proportional to retinal irradiance or power density for a particular chorioretinal pigmentation, exposure duration, spot size, and laser photocoagulator wavelength. TTT uses large spot sizes to produce low retinal irradiances and temperature increases. Its sub-threshold nature is potentially a therapeutic advantage. However, it is also a practical disadvantage, because smaller lesions produced by TTT are not readily detectable. Hence, if retinal irradiance is insufficient to produce a therapeutic temperature increase, this may not be readily detected by medical personnel. Evaluating different aspects of the effect of thermal damage on the RPE and neural retina could improve the reproducibility of sub-threshold photocoagulation results.
Retinal tissue temperature can be monitored by either non-invasive or invasive techniques. Non-invasive techniques include computationally estimating the temperature using proper mathematical models, or employing x-ray or nuclear magnetic resonance (NMR) instrumentation.
Existing commercial instrumentation typically employs mathematical modeling, and calculates the retinal temperature during treatment using a simple model [see, e.g., Mainster et al., Transpupillary thermotherapy for age-related macular degeneration: principles and techniques, Semin. Ophthalmol. Vol. 16 no. 2, pp. 55-59 (2001).] In performing laser eye therapy, in some cases several treatment parameters are adjusted based on the experience and knowledge of the surgeon, technician, or other medical operator. These approaches suffer from substantial inter-subject and intra-subject variability.
Analysis of the images obtained by x-ray and NMR techniques provide a measure of the temperature of the irradiated ocular tissue [see, e.g., Parel et al., U.S. Pat. No. 6,684,097, January 2004]. However the required instrumentation is expensive and may present risks to the patient.
Optical techniques have potential advantages as compared with techniques such as mathematical modeling or characterization by x-ray or NMR. Optical techniques have the potential to perform direct temperature measurement of retinal tissue close to the laser-treated area.
Efforts have been made to use optical methods to measure the retinal temperature during laser treatment. The systems so far proposed include: (i) the optoacoustic techniques; (ii) low coherence interferometry techniques; (iii) interferometric techniques; and (iv) spectral analysis of backscattered light. However, these existing optical techniques typically do not provide a robust, accurate, and reliable determination of the end point of laser treatment that is suitable for use in conjunction with clinical laser eye treatment.
Optoacoustic methods are discussed in Shoule et al., Noninvasive temperature measurements during laser irradiation of the retina with optoacoustic techniques, Proc. SPIE Vol. 4611, pp. 64-71 (2002). In the technique there disclosed, laser-induced pressure waves are generated by interaction of laser irradiation with retinal tissue. A maximum peak of the pressure is proportional to the laser intensity and under certain conditions depends on the temperature of the irradiated tissue. The technique is invasive insofar as an acoustic transducer is placed in physical contact with the patient's eye. The transducer can be integrated into a contact lens which is used in the therapy. However, different contact lenses are used for different procedures and/or patients, and the acoustic transducer should be optimized as a function of lens characteristics. Yet another disadvantage is that precise calibration is required for each specific lens-transducer configuration.
Low-coherence interferometry techniques have been proposed by Lanzetta et al., U.S. Pat. No. 6,540,391, April 2003, for recording lesion formation during ocular laser photocoagulation. This technique does not directly measure retinal temperature, but rather detects morphological changes induced in response to the interaction of ocular tissue with the laser beam. Another interferometric approach is disclosed in Vasyl Molebny, Proc. SPIE vol. 5086, pp. 229-35 (2003). This technique also does not directly measure retinal temperature, but rather measures topological tissue changes during tissue heating. The increase in temperature of the irradiated ocular tissue leads to dimensional changes, and thus to the changes in the topography of eye bottom that can be detected by a double-beam interferometric technique. Such topological changes are expected to be small, however, and moreover correlating such dimensional changes with retinal temperature may be difficult.
Spectral analysis of scattered light has been used to monitor heat-induced sub-cellular structural changes of a human retina, as disclosed in Schuele et al., Noninvasive determination of temperature-induced sub-cellular changes in RPE using light scattering spectroscopy, presented at the SPIE conf. 2004 Photonics West, PW04B-BO11-50. Results were observed in-vitro on single layer of human retinal pigment epithelial (RPE) cells on a glass slide. Strong spectral changes of the backscattered light with temperature were observed for temperature changes of around 25-50° C. This method may be susceptible to inter-subject variability induced by differences in pigmentation between subjects. Moreover, changes in the tissue vascularization could also induce spectral changes in the backscattered light which are unrelated to retinal temperature.
The present disclosure provides improved apparatuses and methods that overcome the above-mentioned limitations and others.